Add user callback for handling invalid characters.

Inspired by ICU, something like:

``` javascript
  buf = iconv.encode(str, 'win1251', { invalidCharHandler: function(char) {
    // Here you can either throw exception which will be propagated outside,
    // or return a replacement char.
  });
```

Probably, some default handlers would also be nice (always throwing, always returning '?', trying to transliterate).
